President Bashar al-Assad has dismissed criticism of Syria's presidential election as voters cast ballots in government-held areas despite Western accusations that it was "neither free nor fair".

"Your opinions have zero value," Mr Assad said, after voting with his British-born wife Asma in the Damascus suburb of Douma, a key rebel stronghold until its recapture by government forces three years ago.

On the eve of the election, the United States, the UK, France, Germany and Italy said the poll was "neither free nor fair", and Syria's fragmented opposition has called it a "farce".

But they will watch powerless as few doubt President Assad will trounce his two virtually unknown challengers to win a fourth term, in a war-battered country mired in economic crisis.

The controversial vote is the second election since the start of a decade-long civil war that has killed over 388,000 people and displaced half the pre-war population.

Huge election posters glorifying Mr Assad have mushroomed across the two-thirds of the country under his control.

State media showed long queues forming outside polling stations in the morning, and later announced that they would remain open for five hours past the planned closing time.

Hundreds of students clamoured to vote outside Damascus University.

"I came to vote for Bashar al-Assad because he is the only man who withstood 10 years of war," said 26-year-old Kinan al-Khatib.

Around him, students chanted: "With our blood and with our souls, we sacrifice our lives for you, Bashar."

The result is not expected before Friday evening.

The election takes place amid the lowest levels of violence since 2011, but with an economy in free fall.

More than 80% of the population live in poverty, and the Syrian pound has plunged in value, causing skyrocketing inflation.

President Assad's campaign slogan, "Hope through work", evokes the colossal reconstruction needed to rebuild the country, requiring billions of euro in funding.

Mr Assad, a 55-year-old ophthalmologist by training, was first elected by referendum in 2000 after the death of his father Hafez, who had ruled Syria for 30 years.

Standing against him are former state minister Abdallah Salloum Abdallah and Mahmud Merhi, a member of the so-called "tolerated opposition", long dismissed by exiled opposition leaders as an extension of the regime.

President Assad has refrained from holding campaign media events and interviews.

But he issued a general amnesty for thousands of prisoners this month, on top of a series of decrees aimed at improving economic conditions.

Last week, thousands of Syrians abroad with the right paperwork cast early ballots in their embassies.

But those who had fled the country illegally - and so who could not show an exit stamp in their passport - were barred from voting.

Several countries that oppose Mr Assad blocked the vote altogether, including Turkey and Germany, which host large Syrian refugee populations.

Syrians cast their votes at a polling booth in Damascus

France's Foreign Minister Jean-Yves Le Drian said the vote was undermined by the fact that many Syrians at home and abroad were unable to take part.

"These elections are not real elections," he said.

The United Nation's Syria envoy, Geir Pedersen, said the polls were being held under the auspices of the current constitution and "not part of the political (transition) process called for in Security Council Resolution 2254" of December 2015.

"What is required is a Syrian-led and -owned political solution, facilitated by the United Nations and backed by constructive international diplomacy," he said.

In rebel-held northwestern Syria, home to three million people, hundreds took to the streets to protest today.

They carried posters saying: "No legitimacy for Assad and his elections."

Protestor Yaqin Beedo called the vote a "sham".

"It is a lie that regime loyalists actually believe," she said.

Protests also broke out in Daraa, only parts of which are under the regime's military control and where some residents called for a boycott.

Kurdish authorities, who have carved out an autonomous zone in the northeast, said they were "not concerned".

In the last multi-candidate poll in 2014, President Assad won 88% of the vote.

It took place as the war raged, with the air force bombing rebel areas of second city Aleppo and fierce fighting in Hama, Idlib and Daraa, as well as near Damascus.

Don't cut Syria cross-border 'lifeline' - UN aid chief

United Nations aid chief Mark Lowcock has appealed to the Security Council not to cut a cross-border aid "lifeline" to millions of Syrians in the country's north as veto-power Russia questioned the importance of the long-running operation.

"We want to see both more cross-line and more cross-border assistance. The cross-border operation - which is a lifeline for more than three million people - cannot be substituted," Mr Lowcock told the council.

"We look to this council to ensure that that lifeline is not severed."

The 15-member Security Council first authorised a cross-border aid operation into Syria in 2014 at four points.

Last year, it reduced that access to one crossing point from Turkey due to opposition from Russia and China over renewing all four.

Another showdown is likely over the renewal of the mandate for the operation, which expires on 10 July.

A resolution to extend council approval needs nine votes in favour and no veto from any of the five permanent members Russia, China, the United States, France and the UK.

Several council members are pushing to increase the number of cross-border aid access points.

"If we don't, people will die. It is that simple," deputy US Ambassador to the United Nations Richard Mills told the council.

"One crossing point alone cannot meet the mass needs of the Syrian people."

Russia's deputy UN Ambassador Dmitry Polyanskiy accused his Western counterparts of disregarding the importance of cross-line aid deliveries from Damascus and "making it obvious that they do not intend to take any steps that would create problems for the fighters holed up in Idlib.

"This hypocritical presentation of the issue is something that we cannot agree with. We are obviously going to have to take into consideration when taking a decision on extending the cross-border mechanism," he said.

In the past decade, the council has been divided over how to handle Syria, with Syrian ally Russia and China pitted against Western members.

Russia has vetoed 16 resolutions related to Syria and was backed by China for many of those votes.